Rwanda's president said the international community "failed all of us", as he marked 30 years since the 1994 genocide that killed around 800,000 people.
President Paul Kagame addressed dignitaries and world leaders who had gathered in Rwanda's capital, Kigali, to commemorate the bloodshed.
"Rwanda was completely humbled by the magnitude of our loss," he said.
"And the lessons we learned are engraved in blood."
On this day in 1994, extremists from the Hutu ethnic group launched a 100-day killing spree, in which members of the Tutsi minority and Hutu moderates were slaughtered.
In a speech later, Mr Kagame thanked fellow African countries including Uganda, Ethiopia and Tanzania for their assistance in accepting Tutsi refugees and ending the genocide.
"Many of the countries representing here also sent their sons and daughters to serve as peacekeepers in Rwanda," he said.
"Those soldiers did not fail Rwanda. It was the international community which failed all of us. Whether from contempt or cowardice."
The genocide was sparked on the night of 6 April 1994, when Hutu President Juvenal Habyarimana was assassinated - the plane he was on was shot down.
Hutu extremists blamed the Tutsi RPF rebel group, and launched a well-organised campaign of slaughter.
Their victims were shot, beaten or hacked to death in killings fuelled by vicious anti-Tutsi propaganda spread on TV and radio.
Thousands of Tutsi women were abducted and kept as sex slaves.
After 100 days of violence, the RPF rebel militia, led by Mr Kagame, succeeded in overthrowing the Hutu authorities and ending the genocide.
Human rights groups say RPF fighters killed thousands of Hutu civilians as they took power - and more after they pursued Hutu militia members who had fled into the Democratic Republic of Congo. The RPF denies this.
Scars from the violence still remain, and new mass graves are still being uncovered around the country.
In the months that followed the genocide, the International Criminal Tribunal for Rwanda was set up in Tanzania.
Dozens of senior officials in the former regime were convicted of genocide - all of them Hutus.
Within Rwanda, community courts, known as gacaca, were created to speed up the prosecution of hundreds of thousands of genocide suspects awaiting trial.
According to Rwanda, hundreds of suspects remain at large, including in neighbouring nations such as DR Congo and Uganda.
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